In Iron Man 3, Tony Stark has a problem powering the Mark 42 suit. There is a key scene where he has a panic attack because the young boy says the suit is not charging. There is also a scene where he is charging it from a car battery.

I've never understood this, as one of the major plot points of Iron Man 2 was Tony developing a new element for the arc reactor which could safety power his Iron Man suit and electromagnet in his chest without killing him.

Tony has the modified reactor in his chest since you can see the triangular shape through his shirt. So why is it suddenly a problem powering the suit?

1 Answer
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Basically the answer is that the newer suits are not powered by the Arc reactor in Tony's chest, but have their own power supply, so that they can fly and fight even if Tony is not in them.

The user Thaddeus has written an excellent and extensive answer on this over at scifi.stackexchange:

None of the new armors being developed used the internal power supply from Tony's internal arc reactor. All of his more recent suits (the ones he made AFTER New York) ran on their own independent power supplies. Note the fact they flew, used repulsors and fought Extremis soldiers whether Stark was in them or not.

The complex design of the Mark 42 made for some unique properties:

Each piece of independent armor had to have its own power supply to utilize the repulsor/anti-grav flight capacity.

Each piece of armor had to be able to independently and cooperatively know where it needed to be and in what order it needed to arrive to make the suit viable upon receipt. This mean they were all capable of managing their own power resources.

Each unit is capable of functioning independent of any other pieces (see Tony's unconventional one hand, one foot aerial ballet of destruction) and thus they are likely not able to be easily recharged unless the suit is in one piece.

The Mark 42 did not seem to be as physically strong as some of the other designs, possibly because of its very modular nature. Since the suit was designed to fit more than Stark, we see Pepper using it quite well, it makes sense to have the suit function without a direct link to the Arc reactor in Tony's chest.

It also makes sense to allow the suit to be recharged on ordinary electricity in the event Stark is not around (as it proved to be a useful feature). Recharging on ordinary electricity was a feature of the comic version of Iron Man from the very first designs.

His suits were designed to absorb solar energy constantly, absorb some electromagnetic energy from his enemies or to be powered directly from land-based power supplies. He could even hook his armor to land based power supplies to augment his strength briefly.